THE ORDOVICIAN PERIOD 



435 



some being straight (Fig. 407 D), some curved (Fig. 407 B> C), and 

 some tightly coiled (Fig. 407 A). The straight forms, represented by 

 Orthoceras (Greek, orthos, straight, and ceras, a horn), were most char- 

 acteristic of the period, some (Endoceras) attaining a length of ten or 

 more feet and a maximum diameter of about one foot. At the other 

 extreme were some less than an inch in length and one eighth of an 

 inch in diameter. Cephalopods have been called the scavengers of 

 the Ordovician, and they were probably the most powerful animals 

 then living. The great diversity of the Ordovician cephalopods is 

 evidence that the group began in the early Cambrian. 



•Crustacea 



Trilobites. — The rapid evolution of the trilobites noted in the dis- 

 cussion of the Cambrian was continued in the Ordovician, during which 



Fig. 408. — Ordovician crustaceans. Trilobites: A, Ceraurus pleurexanthemus ; 

 B, Bathyurus longispinus ; C, Isotelus gigas (greatly reduced) ; D, Trinucleus {Crypto- 

 lithus) tessellatus; E, Bumastus trentonensis ; F, G, rolled and straight specimens 

 of Calymene callicephala; H, I, two views of a rolled specimen of Thaleops ovata. 

 Ostracod : /, Leperditia inflata. 



